Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, and a potential for surge. As such, surge can lead to NVH issues such as undesirable noise from the engine intake system. For example, during hard surge, the compressor allows air to momentarily backflow through the compressor resulting in rapid pressure oscillations, while during soft surge, smaller pressure oscillations are experienced.
To address either form of compressor surge, engine systems may include a compressor recirculation valve coupled across the compressor to enable rapid decaying of boost pressure. One example of such a compressor recirculation valve is shown by Blaiklock et al. in US 2012/0014812. Therein, the compressor recirculation valve is an open/closed type of valve that is maintained closed during steady-state engine operation and actuated open in response to any indication of surge. By opening the valve, a portion of air discharged from the compressor is recirculated to the compressor inlet.
However the inventors herein have identified potential issues with such an approach. As one example, the valve of Blaiklock may be ineffective for managing soft surge. This is due, in part, because of the relatively smaller instability of compressor operation during soft surge. In particular, unlike hard surge, soft surge tends to occur during otherwise steady-state conditions when compressor speed lines on a compressor map have a positive slope. In this region of the compressor map, soft surge generates relatively small amplitude oscillations in pressure and flow. As a result, soft surge is typically not addressed by opening a compressor recirculation valve but by lowering the engine torque curve such that engine operating conditions lie outside of the areas of the compressor map where soft surge tends to occur. As such, this can lead to compromises in driveability and performance, particularly for highly downsized engine configurations and/or at high altitudes and ambient temperatures. On the other hand, if a compressor recirculation valve were opened in response to soft surge, driver torque demand could not be maintained.
In one example, some of the above issues may be addressed by a method for a boosted engine comprising: adjusting flow through each of a first higher flow compressor recirculation path and a second lower flow compressor recirculation path based on a throttle mass flow to maintain a compressor flow rate at or above a threshold flow rate, the second recirculation passage positioned in parallel to the first recirculation passage. In this way, a compressor flow rate can be maintained above a flow rate at the hard surge line and compressor operation may be kept outside a surge region during steady-state as well as transient engine operating conditions.
In one example, an engine system may include a compressor having each of a first compressor recirculation path and a second compressor recirculation path coupling an outlet of a charge air cooler to the compressor inlet. In alternate embodiments, at least one of the recirculation paths may couple an outlet of the compressor to the compressor inlet. The first and second recirculation paths may be positioned in parallel to each other across the compressor. Flow through each recirculation path may be controlled via respective on/off valves. In addition, the second recirculation path may include a flow restriction, in the form of an orifice or a venturi for example, such that compressor recirculation flow through the second recirculation path is at a lower flow rate than compressor recirculation flow through the first recirculation path. An engine controller may be configured to continually adjust a position of the first and second valves, during steady-state and transient engine operating conditions, based on changes in air flow through an intake throttle so as to maintain a compressor flow rate at or above a surge constrained flow rate (that is, a compressor flow rate at a surge limit (e.g., hard surge line) of the compressor). In one example, the throttle mass flow rate may be determined based on the output of a manifold airflow (MAF) sensor, while the surge constrained compressor flow rate is determined based on the compressor surge limit.
If the throttle mass flow is higher than the desired (surge constrained) compressor flow rate, one or more of the compressor recirculation valves may be maintained closed. For example, both valves may be held closed to decrease recirculation flow. Alternatively, the first valve may be held closed while the second valve is left open so that a smaller portion of compressor recirculation flow is kept to provide an improved surge margin. If the throttle mass flow suddenly drops (e.g., during a tip-out) or if the steady-state throttle mass flow is lower than the desired compressor flow rate, both valves may be opened to increase the recirculation flow. Alternatively, the first valve may be opened while the second valve is left closed so that there is a larger portion of compressor recirculation flow. In either case, the flow rate through the compressor is increased, improving the surge margin.
In this way, by adjusting flow through both the compressor recirculation paths based on a throttle mass flow, a compressor flow rate can be kept sufficiently high. This enables compressor operation during steady-state as well as transient operating conditions to remain outside a surge region (e.g., outside a hard surge region and a soft surge region). As such, this allows both hard surge and soft surge to be better addressed. By improving the surge margin under all engine operating conditions, surge related NVH issues and component damage issues are reduced. In addition, soft surge can be addressed without degrading driveability and engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.